Intercostal, diaphragm, leg and arem muscles obtained from non- surviving human fetuses will the examined with the electron microscope to look for the sequence of fine structure changes involved in development of traids and fibrils. The development of triadic junctions in fetal rat, monkey and human muscle fibers will be compared. The effects of puromycin on development of triadic junctions and Z lines will be studied in muscle from newborn and postnatal rats. Further observations will be made on the association of SR with the M line in muscle fibers from primitive vertebrates and certain invertebrates.